Liquid crystals consisting of mixed cholesteric esters are known to undergo color changes at very specific temperatures in the neighborhood of normal room temperature or body temperature. Consequently, such liquid crystals have been found to be very sensitive indicators of small differences in temperature. This has permitted the liquid crystal materials to be used as extremely accurate indicators of locations where temperature differences exist, so as to give visual indication of the location of discontinuities such as flaws in articles supplied with heat at one side, or the location of zones of differing heat capacity or differing vascularity in living tissues.
The practical use of liquid crystals has been somewhat inconvenient in that successful surface contact has required application of the liquid crystal material to the surface to be examined in liquid form, and has also required removal of the liquid crystal material by some kind of washing operation after the examination is completed.
The object of this invention, accordingly, is to provide liquid crystal materials in a solid form which can be applied in continuous intimate contact with surfaces to be examined in a very simple way, and can be removed after the examination is completed without requiring any elaborate cleanup procedure.